1.7 Protons, neutrons and electrons
6 exam questions worth 12 marks. Write your answer, open the mark scheme, then give yourself the marks you earned. Your answers are saved on this device.
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Give the relative charge and relative mass of a proton, a neutron and an electron.
Mark scheme
- proton +1, 1 (1)
- neutron 0, 1 (1)
- electron −1, very small / negligible (1)
Model answer
Proton: charge +1, mass 1. Neutron: charge 0, mass 1. Electron: charge −1, very small mass.
An atom of potassium has an atomic number of 19 and a mass number of 39.
How many protons, neutrons and electrons does it have?
Mark scheme
- protons 19 (1)
- neutrons 20 (1)
- electrons 19 (1)
Model answer
Protons = 19, electrons = 19, neutrons = 39 − 19 = 20.
Explain why atoms have no overall charge.
Mark scheme
- number of protons equals number of electrons (1)
- positive and negative charges cancel / are equal and opposite (1)
Model answer
Atoms have equal numbers of protons and electrons, so the positive and negative charges cancel out.
A magnesium ion is \mathrm{Mg^{2+}}. Magnesium has atomic number 12.
How many electrons does the ion have? Explain your answer.
Mark scheme
- 10 (1)
- atom has 12 electrons and loses 2 to form 2+ ion (1)
Model answer
The atom has 12 electrons; the 2+ ion has lost 2, so it has 10 electrons.
The radius of an atom is about 1 × 10-10 m. The nucleus is about 10 000 times smaller.
Estimate the radius of the nucleus.
Mark scheme
1 × 10-14 m
Model answer
1 × 10-10 ÷ 10 000 = 1 × 10-14 m.
Where is most of the mass of an atom?
Choose one answer.
Correct Not quite: the answer is In the nucleus
Mark scheme
A
Model answer
A: in the nucleus, which contains the protons and neutrons.
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